Soil and habitat complexity effects on movement of the entomopathogenic nematode Steinernema carpocapsae in maize

Habitat heterogeneity enhances conservation of aboveground biological control organisms in agricultural systems. Complex habitats may also conserve beneficial organisms in the soil, although this has rarely been examined in field experiments. We compared the effect of simple (maize) and more complex (maize plus mixed annual plant refuge) habitats on the persistence and dispersal of the entomopathogenic nematode Steinernema carpocapsae applied to soil as nematode-killed insects. The experiment was conducted twice, in 2005 and 2006. We quantified S. carpocapsae dispersal by bioassay of soil samples collected at distances up to 3 m away from the application point within and between crop and refuge habitats. We detected S. carpocapsae in 1.4% (2005) and 0% (2006) of control site samples. S. carpocapsae detection at the source was not affected by habitat complexity but was associated with soil bulk density, plant density, and soil matric potential. The maximum movement rate was 33.3 cm/day, 9 days after application, which exceeded previously reported rates of 7.5 cm/day. S. carpocapsae detection decreased with increasing distance away from the application point. In 2005, soil moisture affected dispersal such that S. carpocapsae were detected further away in complex habitats, when the soil moisture in this habitat was higher. In 2006, movement was similar in both habitats, likely due to similarities in overall plant density in both treatments that year. Our results indicate that movement of S. carpocapsae is not necessarily dependent on habitat complexity but may respond to variation in factors associated with overall plant density, and subsequently, soil moisture.